Anything which is warm will glow, if it is very hot it will glow in visible light , but even at body temperature you are glowing with invisible infra red light. If you have ever watched a police show, you have probably seen footage from thermal cameras, which detect this light.
This glow isn't only a problem for criminals, but also for the military as a beautifully camouflaged tank in conventional light will still look just like a tank in the thermal infra-red.
A part of BAE systems based in Sweden has developed a new form of camouflage which might help. It does this by covering your tank with lots of electrical heat pumps which I assume pump heat from or to the air which is transparent, so doesn't glow when hot. These pixels can heat or cool the surface of the tank very rapidly, and the temperature of each of the pumps can be indivdually be set so they work like pixels.
This means that the pixels can be set to mimic the temperature and pattern of the landscape behind the tank, or as this will never be perfect, it can be set to make a tank pretend to be a car or any other object.
They also can make the pixels much larger for something like a ship when there is no point trying to make it invisible close to, and apparently the system is fairly robust, and actually add to the armour of the vehicle.